Cellular board refers to a structure known as such, which is formed by two substantially parallel surface plates and a core arranged between them. Typically also the core is of plate-like material, but arranged with a form having a direction different from that of the surface plates, for instance by forming folds and grooves between them in the plate material. Typically the core comprises a number of adjacent and parallel straight forms, generally extending substantially over the whole cellular board. The longitudinal direction of the straight forms of the cellular board's core is in this text called the core direction. The cellular board resists bending particularly well in the direction perpendicular to the core direction. The core of prior art cellular board is typically a structure, which is originally separate from the surface plates, but firmly fastened to the surface plates. Typically the surface plates and the core are welded to each other, for instance with laser welding or spot welding. It is also known to glue the surface plates and the core to each other. In a cellular board the surface plates and the core are typically made of metal, e.g. stainless steel or aluminium, but also other materials may come into question. The thickness and the material of the surface plates and the core, and the shape of the core can be dimensioned to suit each situation. The cellular board structure can provide a structure, which is substantially lighter and more rigid, and which better resists bending than a homogenous board structure. The shape of the core strongly influences the rigidity and the strength of the cellular board. A core made of steel can for instance be made of plate, which is bent in a wave-shape where the crests of the waves are typically welded to the surface plates. The cores can also be arranged for instance into the form of the letter V, or they can be formed by plates, which are arranged substantially perpendicular to the surface plates, i.e. in the form of the letter I. The core can be formed by a plate, which is bent in a honeycomb-shape. Bars in the form of tubes with a circular cross-section or other forms can be used as a core.
A problem of traditional cellular board solutions is that it is difficult to fasten the cores of the cellular boards to the surface plates, particularly at the central part of boards with a large area. The manufacture has required expensive equipment. One problem of traditional cellular boards has been that generally only cellular boards of a certain size could be easily made of the same raw materials.
Base floors of buildings are often built of concrete. A base floor is generally supported onto horizontal beams, for instance onto concrete beams. If vertical piles, such as steel piles are used to support the base floor it has required special solutions to fasten the piles. Horizontal piles also required particular construction solutions. The insulation of concrete base floors has also required their own special solutions.